Nickel hydroxide or compositionally modified nickel hydroxide is known to be used as cathode active material for a number of alkaline rechargeable batteries including Ni—Zn, Ni—Cd, Ni—H2, and Ni/MH batteries. Among these batteries, Ni/MH battery has the highest energy density. However, current Ni/MH batteries lose market share in portable electronic devices and the battery-powered electrical vehicle markets to the rival Li-ion technology due to limited gravimetric energy density (<110 Wh kg−1). As such, the next generation of Ni/MH batteries is geared toward improving two main targets: raising the energy density and lowering cost.
As with electrode formation, the properties of nickel hydroxide also differ widely depending upon the production method used. Generally, nickel hydroxide is produced using a precipitation method in which a nickel sulfate and a solution are mixed together followed by the precipitation of nickel hydroxide. The resulting particles are typically of uniform constitution throughout the particle material, but may be of relatively lower packing density limiting their use.
In order to produce high density, substantially spherical particles, nickel hydroxide crystals are grown relatively gradually under carefully controlled process conditions. A nickel salt was grown in an environment stabilized by an ammonium ion. The nickel salt forms complex ions with ammonia to which a strong base is added. The nickel hydroxide is then gradually precipitated by decomposition of the nickel ammonium complex. One drawback of this general method is that the reaction rate is difficult to control, so tailoring methods have been introduced to separate critical reaction steps in the production process to compensate for said difficulties. For example, U.S. Pat. No. 5,498,403, entitled “Method for Preparing High Density Nickel Hydroxide Used for Alkali Rechargeable Batteries”, issued to Shin on Mar. 12, 1996, discloses a method of preparing nickel hydroxide from a nickel sulfate solution using a separate or isolated amine reactor. Nickel sulfate is mixed with ammonium hydroxide in the isolated amine reactor to form nickel ammonium complex. The nickel ammonium complex is removed from the reactor and sent to a second mixing vessel or reactor where it is combined with a solution of sodium hydroxide to obtain nickel hydroxide. Such a method relies heavily on a raw material source of very high purity.
Another method of producing nickel hydroxide particles results in a multi-layered particles such as particles of a core-shell structure whereby an electrochemically active core is surrounded by an active outer layer. The outer layer may have a three dimensional structure that will allow sufficient ion conduction to allow ions to reach the inner active particle material. An example of particles with this structure is found in U.S. Pat. No. 6,416,903. Methods of making such core/shell structures can be achieved using one or more reactors whereby an active seed is first formed and then subjected to a second precipitation reaction to create a second material disposed about the seed. In this way, particles can be created whereby the shell material will impart improved cycle life and the core will impart high-temperature performance to the overall material.
While these and other methods of forming core/shell particles for metal hydride battery systems, there remains a need for improved capacity of the resulting materials while maintaining or improving cycle life. As will be explained herein below, the present invention addresses these needs by providing new materials that may be made in a single stage reactor and have properties improved over traditional core/shell particles. These and other advantages of the invention will be apparent from the drawings, discussion, and description which follow.